Field of the Invention
The present disclosure relates to a driving apparatus and a stepping motor for controlling speed and position.
Description of the Related Art
The stepping motor rotates by sequentially switching current which flows into a coil of the motor. At this time, feedback control of a rotation speed or a rotation position is not required. The stepping motor is capable of rotating by a fixed angle, including a mechanical angle of 1.8°, 7.5° and the like in a predetermined motor structure. Therefore, it is noted that the rotation angle is guaranteed to have a predetermined accuracy. Practically, the current is switched using a general-purpose stepping motor driver IC. In particular, using the stepping motor driver IC, a pulse voltage corresponding to a rotation angle desired to advance (i.e. desired to rotate) is input in a predetermined frequency to control the rotation speed and the rotation angle. The stepping motor which is controllable in this way has an advantage that it can perform easy and reliable operation.
On the other hand, the stepping motor has some disadvantages, which are stepping-out and problems caused by step operation, for example. Stepping-out of the stepping motor is a phenomenon in which, when a load torque exceeds an output torque of the stepping motor, motor rotation is stopped, which interrupts continuous rotation of the stepping motor. Such a stepping-out phenomenon should never be present in a driving apparatus. Therefore, the current value that flows into a coil of the stepping motor is determined to obtain the output torque in which some margin is further added to a maximum load torque such that the output torque always exceeds against a conceivable load torque. However, the stepping motor is maintained, during most of its operation time, in a state in which a small load torque is applied. The current determined as above causes some extra current in the above state. As a result, power loss is increased, which is a disadvantage as compared to a case where a DC motor is used in light of power saving.
Also, as a nature of the stepping motor, it is rotated by step operation. Therefore, there are problems in that vibration, noise, torque ripple or speed unevenness becomes large. For such problems, a waveform of current flowing into a coil is brought close to a sine wave through microstep driving from a rectangular wave (i.e., two phase excitation drive) to improve a conventional stepping motor. Adapting the microstep driving indeed enables to reduce torque ripple, speed unevenness, and vibration. However, the stepping motor is still controlled by the open-loop system, and the position and speed are controlled by a step angle. That is, to adapt the microstep driving does not directly contribute to reducing power loss.
Then, as a method to make use of the advantage and reduce the disadvantage of the stepping motor, a feedback control using a sensor (for example, an encoder) which detects rotation speed and position of the stepping motor is proposed.
There are two major types of feedback control system of the stepping motor. One is a system to control motor current such that torque which can resist a load torque is generated while constantly keeping phase relation between a field magnetic flux and a motor current. The system is the same as that of a brushless motor so that it is called a brushless driving system, for example. The other is a system to control the phase angle of a field magnetic flux and a motor current while constantly keeping a magnitude of the motor current. The system is called an advanced angle control system, for example. Here, the motor current indicates the current vector in which currents of each coil of the stepping motor are combined. In both feedback systems, the phase relation between the field magnetic flux and the motor current needs to be determined. Therefore, detection of a magnetic pole position is necessitated.
The encoder mounted to a rotation shaft of the stepping motor works as rotation position detection sensor. The encoder detects a rotation angle to a certain reference position (initial position). However, there is not any positional correlation between the reference position set in the encoder and a magnetic pole position of a stepping motor rotor. As a result, the magnetic pole position cannot be detected. Therefore, it is necessary to associate the reference position of the encoder with the magnetic pole position.
Conventionally, to determine the magnetic pole position, a constant current flows into a motor coil when the stepping motor is stopped. Then, a stop position where a rotor is held in this state is detected. Thereafter, the detected magnetic pole position is associated with an angle of the encoder. The principle will be described with reference to FIG. 7. FIG. 7 is a characteristic diagram illustrating the relation between an electrical angle and torque in the stepping motor. The characteristic diagram shown in FIG. 7 indicates that a rotor rotates with respect to a stabilizing point at which torque is 0 Nm. That is, FIG. 7 shows that torque is generated in a sine wave form in accordance with a change of a magnetic pole position. As shown in FIG. 7, the stepping motor generates the maximum torque when the electrical angle is ±90°. The torque value is represented by an expression (1) as:T=Kt*I*sinq  (1);
where T represents torque, Kt represents torque constant, I represents current value, and q represent electrical angle.
The conventional method assumes that there is no-load or negligibly small load. This causes a contradiction in that, if torque is generated in the stepping motor when it is stopped, the rotor rotates. Therefore, assuming that the magnetic pole is positioned at the electrical angle of 0° when the stepping motor is stopped and associating a position detected by the encoder with the magnetic pole position, the magnetic pole position was determined.
In an actual stepping motor driving apparatus, however, load torque by a static frictional force caused by a last rotation driving sometimes remains. The stepping motor is stopped with the load torque being balanced with the torque generated by the stepping motor. As a result, in the conventional technology, affected by the load torque, an initial magnetic pole position may incorrectly be detected. For example, as shown in FIG. 7, the generated torque is balanced with the electrical angle of θ1 when the load torque is T1. If the magnetic pole is associated with the electrical angle of 0° in this state, an angular error of θ1 will be caused.
In view of reducing the occurrence of such errors, a stepping motor driving device, disclosed in US2007/216335A1, is proposed. FIG. 8 is a characteristic diagram illustrating relation between an electrical angle and torque in a stepping motor of the stepping motor driving device disclosed in US2007/216335A1. In FIG. 8, T2 and q2 respectively represents torque and an electrical angle which balance with static frictional force when the stepping motor is stopped in a certain fixed current I. The detection value detected by the encoder in this state is stored as x1. Then, a fixed current which flows into a coil of the stepping motor is changed to a*1 (a≠0). As shown by a chain line in FIG. 8, the torque characteristic at this state is represented by a sine wave in which amplitude is multiplied by a.
Also, if, in that case, the generation torque T2 remains unchanged, the electrical angle is θ3. The detection value detected by the encoder in this state is stored as x2.
Above matters are organized with following expressions.T2=Kt*I*sin θ2  (2)T2=Kt*a*I*sin θ3  (3)θ2−θ3=x1−x2=δ  (4)
The expressions (2), (3), and (4) are solved for θ2 . Then, the following is obtained.θ2 =arctan(a*sin θ/(1−a*cos δ))  (5)
As a result, correction amount of the magnetic pole position based on the detection value x1 can be obtained by the expression (5). A principal point of the proposal is that the encoder is not capable of measuring the absolute value of the electrical angles θ2 and θ3 but is capable of measuring the difference and θ2 can be obtained by calculation.
In the stepping motor driving device as disclosed in US2007/216335A1, a deviation of the magnetic pole position caused by the static frictional force is corrected on an assumption that the remaining load torque T2 is constant. On the other hand, the actual static frictional force generates load torque which balances with the generated torque of the stepping motor until the actual static frictional force exceeds the maximum static frictional force against a change in an external force. That is, there exists some dead zone in which the rotor does not rotate even when the generated torque is changed. As a result, the stepping motor driving device as disclosed in US2007/216335A1 cannot determine (detect) the magnetic pole position with high accuracy, which is a problem.
The present disclosure is to provide a stepping motor driving apparatus which can determine the magnetic pole position with high accuracy even in a case where load torque is generated in a stepping motor when its rotation is stopped.